1. Field of the Invention
The present invention generally relates to grind resins for use in coating compositions. More specifically, the present invention relates to grind resins formulated to minimize color shift of a cured film formed from the coating composition.
2. Description of the Prior Art
Coating compositions are typically applied to substrates to form a cured film that provides the substrates with certain functional and aesthetic qualities, such as protection and color. Examples of substrates that are typically coated with such coating compositions are metal coils, which may be used to form roofing, siding, and rainwear for commercial and residential structures.
Coating compositions typically include resins, solvents, additives, and pigments, which impart color. A pigment dispersion may be used to incorporate the pigments into the coating compositions. The pigment dispersion typically comprises a pigment and a grind resin. The grind resin thoroughly disperses the pigment throughout the pigment dispersion.
A grind mill may be used to integrate the pigment into the grind resin. The grind mill typically integrates the pigment into the grind resin until a desired particle size of the pigment is achieved, and until the pigment is appropriately wetted by the grind resin.
Many prior art grind resins are known including, but not limited to, polyester-modified urethane grind resins, as disclosed in U.S. Pat. No. 3,498,940 to Laganis. Specifically, Laganis discloses a polyester-modified urethane grind resin comprising a reaction product of a dibasic acid, a dimer fatty acid, a diol, and a polyhydric alcohol containing at least three hydroxyl groups.
Many grind resins of the prior art, including those disclosed in Laganis, are inadequate when minimal color shift is desired. Color shift is an undesirable result in coating industries because of noticeable appearance changes in substrates after aging. As is known in the art, color shift is defined as a change in color of a cured film formed from a coating composition after aging over time as compared to a color of a cured film formed from the coating composition before aging and is measured by a spectrophotometer or colorimeter. Many grind resins of the prior art do not respond well to mechanical forces exerted by the grind mill when integrating the pigment and do not effectively wet the pigment to maintain the pigment dispersed throughout the pigment dispersion. Instead, many grind resins of the prior art cause the pigment dispersions to agglomerate or settle, resulting in poor stability and inadequate shelf life. Grind resins with poor stability contribute to color shift of cured films formed after coating compositions including the pigment dispersions are manufactured, applied to substrates, and cured.
Additionally, pigment dispersions comprising grind resins of the prior art may have unacceptable viscosities. Unacceptable viscosities inhibit the amount of pigment that can be integrated into the pigment dispersions. As a result, many grind resins of the prior art often require increased use of solvents and other additives, thereby increasing an overall volatile organic content of the pigment dispersions, which is also an undesirable result in coating industries.
Due to the inadequacies of the grind resins of the prior art, including those disclosed in Laganis, there remains an opportunity to provide a grind resin for use in coating compositions which does not suffer from the aforementioned inadequacies. More specifically, there remains an opportunity to provide a grind resin that effectively wets pigment, adequately disperses pigment throughout pigment dispersions, and minimizes color shift of cured films formed from coating compositions.